The present invention relates to a method of positioning a patient in an x-ray apparatus for dental x-ray radiography. The invention further relates to a dental x-ray apparatus for implementing the method.
In dental radiography, a precise position of the patient's head with respect to the radiological apparatus is mandatory to achieve the best focusing and best image quality. The need for controlling the position of a patient has first arisen in the radiotherapy domain. In this field, the X-ray dose can be greatly increased, up to 10 times, if the position of the patient is known with high accuracy. The increased X-ray dosage allows limiting the exposure of healthy tissue to radiation.
Mechanical Systems and Fixed X-Ray Systems
The first system developed for radiotherapy was based on a stereotactic/head frame. This head frame is based on an abstract model to which the actual head and a CAT model are both referred. The head frame allows patient positioning to be achieved with a high accuracy (from 2.5 to 4.5 mm). However, a helmet has to be worn by the patients, which requires a very careful positioning if large positioning errors are to be avoided. This system seems to transfer the problem of the patient's positioning to the positioning of the helmet.
Active Helmets and Movable X-Ray Systems
An alternative solution which is also based on wearing helmets is based on using instrumented helmets (active helmets). These active helmets are very similar to Head Mounted Displays used in Virtual Reality. The active helmets carry magnetic or ultrasound sensors (magnetic sensors are badly distorted by ferromagnetic material, and ultrasound sensors do not appear to be so accurate). However, the use of a helmet or any similar device is not adequate for dental radiography.
2D Radiographic or 2D Video-Imaging and Movable X-Ray Systems
An alternative solution is to use an intelligent video system that surveys the patient and outputs data required for the correction of the location and orientation of the X-ray system. A simple solution is based on computed tomography (CT) data and the patient can be aligned with the machine using 2D radiographies. This approach requires additional 2D radiographies, and does not meet the needs of dental radiography.
Two of the first attempts to use natural video images are described as follows. In the first system, 2D images of the patient face are aligned with 2D reference images (obtained from CT data). Then, an opportune correlation measure is used for aligning the patient's current images with reference ones. In the second system, 2D images of the patient's face are aligned with 2D images of the patient face in a reference condition. The operator then sees on the display the subtraction (difference) images. For this system, reported alignment errors are in the range 1-3 mm. These methods have been recently extended using three video cameras to acquire 2D images.
3D Video Images and Movable X-Ray Systems
More recently, attempts to use 3D digital models of the face, reconstructed in real-time, have been explored. In one system, the 3D reconstruction of the patient's surface in a current position is performed in real-time. This 3D digital model is then aligned with the model of the patient in a reference condition. These systems require that an accurate digital model of the patient be built in real-time. Although automatic 3D scanners have been on the market for few years (e.g., Minolta), their impact on the costs of an x-ray machine is quite high. Moreover, these systems are difficult to operate automatically for several reasons. First, very little information is available for alignment, i.e, the body shape does not have geometrical features (sharp peaks, valleys, etc.), which would allow defining robust error measures, to be used to evaluate alignment. Second, the 3D models are represented as meshes or surfaces. In the first case, the distance between the mesh and the 3D mesh is required, which is an approximate measurement. In the second case, the surface to surface distance has to be determined, which requires a normal to the surface computation that is time consuming.
A simpler way to build a working 3D model, is to resort to passive retro-reflective markers which are positioned onto the patient. The body CT image is acquired with CT and a 3D approximated model of the body segment can be reconstructed and aligned with CAT data. Alternatively, an active pattern (e.g., a grid, a bar code) can be projected over the patient's body. An evolution of these systems, of particular interest in x-ray radiography, is based on substituting the CAT data with laser markers.
Characteristics of Dental X-Ray Systems and Drawbacks Thereof
Up to now, positioning systems for dental panoramic radiographies are based on mechanical systems combined with laser markers. The patient bites a mechanical device that is aligned with the orthopantomographic machine. A set of laser markers are then projected on the head of the patient. The position of the mechanical device (bite block) and of the patient's orientation is manually adjusted by an operator. This operation mode has two main drawbacks. First, the laser markers are projected on the frontal and lateral parts of the face, so the operator has to move around the patient to see if the markers are properly aligned with the patient's head. Second, once this alignment has been achieved and checked, the operator exits from the room where the orthopantomographic apparatus and the patient are located to start the x-ray imaging process. Unfortunately, during this time period before the operator starts the x-ray imaging process, there is a high likelihood that the patient will move out of the proper alignment position for the x-ray imaging process.
Moreover, the possibilities for an operator of the x-ray apparatus to move within the working space of an orthopantomographic system is very restricted, and a good analysis of the patient alignment can be uncomfortable for the operator. Therefore, if the patient is not aligned with the markers, the operator is forced to assume uncomfortable positions in order to help the patient to reach the required position.
Therefore, what is needed is an easy and reliable system and method for positioning and aligning a patient in an x-ray apparatus for dental radiography.